Ofdm communication system and method having a reduced peak-to-average power ratio

ABSTRACT

An OFDM system embeds sequence information in the transmitted signal that reduces peak average power ratio (PAP) with minimal impact on the overall system efficiency. A marker is embedded onto the transmitted information that is used to identify the combining (inversion) sequence at the receiver. In one embodiment, selected tones in a cluster are rotated when the corresponding phase factor rotates the cluster.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Patent Application Ser. No.13/706,054, filed Dec. 5, 2012, now U.S. Pat. No. 8,761,191, which is acontinuation of U.S. patent application Ser. No. 13/205,231, filed Aug.8, 2011, now U.S. Pat. No. 8,335,225, which is a continuation of U.S.patent application Ser. No. 12/490,050, filed Jun. 23, 2009, now U.S.Patent No. 7,995,602, which is a continuation of U.S. patent applicationSer. No. 11/685,028 filed Mar. 12, 2007, now U.S. Pat. No. 7,558,282,which is a continuation of U.S. patent application Ser. No. 11/190,308filed Jul. 26, 2005, now U.S. Pat. No. 7,206,317, which is acontinuation of U.S. patent application Ser. No. 09/778,254 filed Feb.7, 2001, now U.S. Pat. No. 6,928,084, which claims the benefit of U.S.Provisional Application No. 60/192,708, filed on Mar. 28, 2000. Theaforementioned related patent applications are all herein incorporatedby reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

FIELD OF THE INVENTION

The present invention relates generally to communication systems and,more particularly, to Orthogonal Frequency Division Multiplexing (OFDM)wireless communication systems.

BACKGROUND OF THE INVENTION

Orthogonal frequency division multiplexing (OFDM) wireless communicationsystems have desirable characteristics for high-bit-rate transmission ina radio environment. For example, by dividing the total bandwidth intomany narrow subchannels, which are transmitted in parallel, the effectsof multipath delay spread can be minimized. OFDM systems have beenadopted or proposed for Digital Audio Broadcasting, Digital TerrestrialTelevision Broadcasting, wireless LANs, and high-speed cellular data.

One disadvantage of using OFDM techniques for wireless applications isthe potentially large peak-to-average power ratio (PAP) characteristicof a multicarrier signal with a large number of subchannels. Forexample, a baseband OFDM signal with N subchannels has a PAP equal tothe number of subchannels squared divided by the number of subchannels,i.e., PAP=N²/N=N. For N =256, the PAP≈24 dB. When passed through anonlinear device, such as a transmit power amplifier, the signal maysuffer significant spectral spreading and in-band distortion.

Conventional solutions to reducing the PAP for OFDM systems includeusing a linear amplifier and using a non-linear amplifier while backingoff the amplifier operating point. However, these approaches result in asignificant power efficiency penalty.

Another attempt to reduce the PAP includes deliberately clipping theOFDM signal before amplification to improve the PAP at the expense ofsome performance degradation. Another technique uses nonlinear blockcoding, where the desired data sequence is embedded in a larger sequenceand only a subset of all the possible sequences are used, specificallythose with low peak powers. Using this approach, a 3 dB PAP can beachieved with a relatively small bandwidth penalty. However, toimplement such a coding scheme, large look-up tables are required at thetransmitter and the receiver, thereby limiting its usefulness toapplications with a small number of subchannels. Progress has been madetoward coding schemes that reduce the PAP and can be implemented in asystematic form with some error- correcting capabilities. Nevertheless,these methods are difficult to extend to systems with more than a fewsubchannels and the coding gains are relatively small for adequatelevels of redundancy.

Additional techniques for improving the statistics of the PAP of an OFDMsignal include selective mapping (SLM) and partial transmit sequence(PTS). In SLM, a predetermined number M of statistically independentsequences are generated from the same information and the sequence withthe lowest PAP is chosen for transmission. However, this introducesadditional complexity for providing improved PAP statistics for the OFDMsignal. In addition, the receiver must have knowledge about thegeneration process of the transmitted OFDM signal in order to recoverthe information. The sequence information is sent as side information,resulting in some loss of efficiency.

It would, therefore, be desirable to provide an OFDM system having areduced PAP with optimal efficiency.

SUMMARY OF THE INVENTION

The present invention provides an OFDM system that embeds PAP-reducinginversion sequence information in the transmitted signal with noadditional overhead. With this arrangement, the PAP ratio of thetransmitted signals is reduced with little or no impact on the overallsystem efficiency. While the invention is primarily shown and describedin conjunction with an OFDM system, it is understood that the inventionis applicable to other systems in which it is desirable to detectembedded sequence information.

In one aspect of the invention, in an OFDM system a block of symbols ispartitioned into a predetermined number of clusters. A respective phasefactor is generated for each cluster to form an inversion sequence thatreduces the PAP of the transmitted signals. A variety of techniques canbe used to generate the inversion sequence including suboptimaliterative algorithms and optimum approximations, which can correspond toWalsh sequences for example. The inversion sequence is embedded onto thetransmitted data by rotating selected ones of the tones in a clusterbased upon whether the cluster phase factor rotates the cluster. In oneembodiment, binary phase factors, i.e., plus/minus one, are used. If theinversion sequence does not rotate the cluster, i.e., the cluster phasefactor is plus one, then none of the tones in the cluster are rotated.If the inversion sequence does rotate the cluster, i.e., the clusterphase factor is minus one, then every other tone in the cluster isrotated by a predetermined amount, e.g., π/4 radians.

To detect the inversion sequence, the receiver first removes the datamodulation. A test statistic is then generated for each cluster. Thetest statistics can be used to make decisions in a variety of waysincluding quantizing the test statistic and making independent decisionsfor each cluster, quantizing the test statistics and decoding the entiresequence by nearest Hamming distance, and decoding the sequence bynearest Euclidean distance.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a top level diagram of an OFDM system having reduced PAP inaccordance with the present invention;

FIG. 2 is a pictorial representation of OFDM subcarriers that can begenerated by the OFDM system of FIG. 1;

FIG. 3 is a pictorial representation of orthogonal OFDM subcarriers thatcan be generated by the OFDM system of FIG. 1;

FIG. 4 is a schematic representation of a portion of an OFDM system thatembeds inversion sequence information in the transmitted signals inaccordance with the present invention;

FIG. 5 is a flow diagram of an exemplary sequence of steps for providingan optimal PAP for an OFDM system in accordance with the presentinvention;

FIG. 6 is a graphical depiction showing the probability of error indetecting an inversion sequence embedded in the transmitted signals inaccordance with the present invention; and

FIG. 7 is a graphical depiction of detection performance for an OFDMsystem in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a technique for achieving high-bit-ratewireless data transmission in an orthogonal frequency divisionmuliplexing (OFDM) system with a relatively low peak-to-average powerratio (PAP). The system utilizes partial transmit sequences forproviding favorable PAP statistics with combining sequence informationembedded in the transmitted data. With this arrangement, no overhead isrequired to provide the combining sequence information to the receiver.

FIG. 1 shows an exemplary OFDM system 100 having sequence data embeddedin the transmitted data in accordance with the present invention. Thesystem 100 includes components for transmission and reception of data. Acoding subsystem 102 encodes binary data from a data source. The codeddata is interleaved by an interleaving subsystem 104 and then mappedonto multi-amplitude multi-phase constellation symbols by a mappingsubsystem 106. In one particular embodiment, the multi-amplitudemulti-phase constellation symbols include quadrature phase shift keying(QPSK) symbols. Pilot signals can then be inserted by a pilot insertionsubsystem 108 to estimate the channel at the remote subscriber unitreceivers. A serial-to-parallel conversion subsystem 110 converts theserial data stream to a parallel data stream that is provided to aninverse fast fourier transform (IFFT) subsystem 112.

The transformed data is converted to serial data stream by aparallel-to-serial converter 114. Cyclic extension and windowing can beadded by a subsystem 116 prior to digital-to-analog conversion by a DAC118 and transmission by an antenna 120 system. A receive portion 130 ofthe OFDM system includes corresponding components for extracting thedata from the received OFDM signal.

As shown in FIG. 2, the OFDM system 100 utilizes an overlappingorthogonal multicarrier modulation technique having a plurality ofsubcarriers 150. FIG. 3 shows the orthogonal relationship of thesubcarriers. More particularly, each of four subcarriers 160 a-160 d ofone OFDM data symbol has an integral number of cycles in the interval T.The number of cycles between adjacent subcarriers differs by one.

FIG. 4 shows a portion of an OFDM system 200 that embeds PAP-reducinginversion sequence information within the transmitted data with nooverhead in accordance with the present invention. With thisarrangement, the need to dedicate reference subcarriers, e.g., one foreach cluster, to transmit phase factor information is eliminated.

The OFDM system 200 includes a data source 202 generating a data streamX that is converted from a serial stream to a plurality of parallel datastreams X₁-X_(M) and partitioned by a subsystem 204 as described below.The partitioned data streams are transformed by respective inverse fastFourier transform systems 206 ₁-206 _(M), in a conventional manner. Theclusters of transformed data are rotated by respective phase factorsb₁-b_(m), which are embedded into the transmitted data, as describedbelow in detail. An optimizer subsystem 208 can facilitate selection ofthe phase factors that reduce the PAP ration of the transmitted OFDMsignals.

Initially, a block of N symbols {X_(n), n=0, . . . ,N−1} is formed witheach symbol modulating one of a set of N subcarriers, {f_(n), n=0, 1, .. . , N−1}. The N subcarriers are chosen to be orthogonal, i.e.,f_(n)=nΔf, where Δf=1/NT and T is the original symbol period, as shownin FIG. 3. The resulting signal after D/A conversion can be expressed asset forth below in Equation 1:

$\begin{matrix}{{{x(t)} = {\sum\limits_{n = 0}^{N - 1}\; {X_{n}^{j\; 2\; \pi \; f_{n}t}}}},{0\mspace{14mu} \# \mspace{14mu} t\mspace{14mu} \# \mspace{14mu} {NT}}} & {{Eq}.\mspace{14mu} (1)}\end{matrix}$

The PAP of the transmitted signal from Equation (1) can be defined asshown in Equation (2) below:

$\begin{matrix}{{PAP} = \frac{\max {{x(t)}}^{2}}{E\lbrack {{x(t)}}^{2} \rbrack}} & {{Eq}.\mspace{14mu} (2)}\end{matrix}$

To obtain the partial transmit sequence (PTS), the input data block ispartitioned into disjoint sub-blocks or clusters by subsystem 204 whichare combined to minimize the PAP. A data block is defined as {X_(n),n=0, 1, . . . , N−1}, which can be represented as a vector X=[X₀X₁ . . .X_(n-1)]^(T), where T is the symbol period. The vector X is partitionedinto a predetermined number M of disjoint sets represented by vectors{X_(m), m=1,2, . . . M}. The partial transmit sequence technique forms aweighted combination of M clusters as set forth below in Equation 3:

$\begin{matrix}{X^{\prime} = {\sum\limits_{m = 1}^{M}\; {b_{m}X_{m}}}} & {{Eq}.\mspace{14mu} (3)}\end{matrix}$

where {b_(m), m=1, 2, . . . , M} are phase or weighting factors, whichcan be pure rotations. In the time domain, this relationship can berepresented as shown in Equation 4 below:

$\begin{matrix}{x^{\prime} = {\sum\limits_{m = 1}^{M}\; {b_{m}x_{m}}}} & {{Eq}.\mspace{14mu} (4)}\end{matrix}$

The vector x_(m), which is referred to as the partial transmit sequence,is the Inverse Fast Fourier Transform (IFFT) of vector X_(m). The phasefactors b_(m) are chosen to minimize the PAP of x′, as described below.

The phase factors can be generated in a variety of ways to minimize thePAP of the transmitted OFDM signals, including optimization, iteration,and random generation. For example, a predetermined number of Walshsequences can be generated.

In one particular embodiment, the phase factors b_(m) are binary phasefactors, i.e., ±1. In an alternative, more complex embodiment, the phasefactors include ±1 and ±j. After the input data block is divided into apredetermined number M of clusters, the M N-point partial transmitsequences are formed. For example, an OFDM system having 256 subcarrierscan include sixteen (M=16) data clusters each having sixteensubcarriers.

FIG. 5 shows an exemplary sequence of steps for determining binary phasefactors for the partial transmit sequence. In step 300, the phasefactors b_(m) are set to be one for all m and in step 302 the PAP (PAP₀)of the combined signal with all phase factors set to one is computed. Instep 304, the phase factor index m is set to 1.

In step 306, the first phase factor b₁ is inverted, i.e., b₁=−1, and thePAP is re-computed with inverted phase factor in step 308. In step 310,is it determined whether the new PAP value is lower than the originalPAP₀. If it is lower, then in step 312 the first phase factor b₁ remainsminus one as part of the final phase sequence {b_(m), m=1, . . . , M}.If it is not lower, in step 314 the first phase factor is reset to one,i.e., b₁=1. In step 316, the index value M is examined and in step 318the index value is incremented until each phase factor is determined tobe a one or a minus one.

Alternatively, a predetermined number of random sequences, which can beWalsh sequences, are selected. The information sequence is multiplied bya predetermined number of the sequences. The result providing the bestPAP characteristics is then selected. This approach approximates anoptimum PAP as described in L. J. Cimini. Jr. and N. R. Sollenberger,“Peak-to-Average Power Ration Reduction of an OFDM Signal Using PartialTransmit Sequences,” IEEE Commun. Letts., Vol. 4, No. 3, March 2000, pp.390-393, which is incorporated herein by reference.

FIG. 6 shows the PAP versus CCDF simulated results for an OFDM systemhaving 256 subcarriers with the transmitted signal oversampled by afactor of four. QPSK signal modulation is assumed with the energynormalized to unity. Results are shown for the case of a single OFDMblock (M=1) and for 16 clusters (M=16) each including 16 subcarriers.The unmodified OFDM signal has a PAP that exceeds 10.4 dB for less than1% of the blocks. For the suboptimal algorithm using 16 Walsh sequencesof length 16 as the inversion sequence, a value of about 8 dB isobtained. By using the PTS approach with the optimum binary phasesequence for combining, the 1% PAP reduces to 6.8 dB. While adegradation of about 1 dB is encountered using the suboptimal approach,the optimization process has been reduced to 16 sets of 16 additions, asignificant savings as compared to finding the optimum set of phasefactors.

To recover the data, the OFDM system receiver determines the inversionsequence that was embedded in the transmitted signals. In contrast toknown systems that send inversion sequencing information as explicitside information (via subcarriers) at the expense of some loss inefficiency, an OFDM system in accordance with the present inventionembeds a marker onto the transmitted data that can be used to uniquelyidentify the inversion sequence at the receiver. The detection of theinversion sequence should be sufficiently reliable so as not to have asignificant effect on the overall system performance.

As described above, the OFDM system embeds markers on the transmittedsignals. In an exemplary embodiment described above, if the inversionsequence does not rotate the cluster, e.g., b_(m)=1, then no tones inthe cluster are rotated. If the inversion sequence rotates the cluster,e.g., b_(m)=−1, then every other tone in that cluster is rotated by π/4radians. This arrangement is equivalent to using two signalconstellations for the data symbols in a cluster: one for the unrotatedclusters and another, rotated by π/4, for the modified clusters. Thisalgorithm puts an embedded marker on rotated clusters that can bereliably detected even in the presence of noise and multipath fadingwith a minimal impact on the overall system performance.

To detect the inversion sequence, the data modulation must first beremoved. In an illustrative embodiment, the frequency symbols are raisedto the fourth power, which is a standard approach for removing QPSKmodulation. Higher-order PSK modulations can be removed in a similarfashion, as is well known to one of ordinary skill in the art. With themodulation removed, the data symbols (in the frequency domain) can bedifferentially detected by computing, for each cluster, a test statisticas set forth below in Equation 5:

$\begin{matrix}{Z_{m} = {\sum\limits_{j = 1}^{{N/M} - 1}\; ( {Y_{j,m}Y_{{j + 1},m}^{*}} )^{4}}} & {{Eq}.\mspace{14mu} (5)}\end{matrix}$

where Y_(j,m) represents the jth tone in the mth cluster and * denotesconjugation. Thus, in the absence of noise, if cluster m was not alteredby the inversion sequence, then the mth test statistic Z_(m) is+(N/M−I). If b_(m)=−1, then Z_(m) is −(N/M−1). Therefore, a relativelysimple binary detection scheme can recover the inversion sequence. Thesummation over the tones in a cluster averages the noise and provides asignificant performance improvement.

Given the decision statistic in Equation 5, a variety of detectionschemes can be used. In one embodiment, the test statistic is quantizedto ±1 and decisions are made independently for each cluster. While thisapproach is relatively simple, there is no straightforward mechanism forcorrecting errors.

In another embodiment, detection performance is improved by quantizingthe individual test statistics Z_(m) and then decoding the entiresequence to the nearest sequence, e.g., Walsh sequence. Specifically,the system first generates the sequence {Re[Z_(m)], m=1,2, . . . ,M} andquantizes each component to +1 or −1. The system then chooses the Walshsequence of length M that is closest, in Hamming distance, to theresulting sequence. This technique provides error correction since thereceived sequence is mapped into one of only M possible Walsh sequences.

In a further embodiment that provides further performance improvements,all of the information in the decision statistics is retained.Therefore, one preferred strategy is to compute the sequence {Z_(m),m=I,2, . . . ,M} and then choose the Walsh sequence of length M that isclosest, in Euclidean distance, to the resulting sequence.

The performance of an OFDM system in accordance with the presentinvention was simulated. The simulated OFDM system includes 256subcarriers (N=256), which are divided into 16 clusters (i.e., M=16),each having 16 subcarriers. QPSK is used to modulate the tones.Performance is measured by the word error rate (WER), where wordcorresponds to one OFDM block or, equivalently, the length of oneinversion sequence. Initially, results on the detection performance arebased upon the probability that the inversion sequence is received inerror.

FIG. 7 shows the probability of error in detecting the inversionsequence as a function of the signal-to-noise ratio (SNR) in anadditive, white, Gaussian noise (AWGN) environment. The simplecluster-by-cluster detection scheme requires the highest SNR for adesired WER. By using minimum distance decoding, whether based onHamming or Euclidean distances, significant improvements are obtained.The benefit comes from the error correction that is possible withminimum distance decoding. The 16 Walsh sequences of length 16 have aminimum distance of 8 and, as such, can correct up to 4 errors. Usingthe Hamming (Euclidean) distance, a 1% WER can be achieved with an SNRof about 3.2 dB (2.3 dB).

The present invention provides an OFDM system that provides enhanced PAPstatistics with minimal loss in efficiency. The system embeds combiningor inversion sequence information without additional overhead. Inaddition, the embedded inversion sequence can be reliably detected bythe receiver. Performance can be further improved by increasing thenumber of tones per cluster.

One skilled in the art will appreciate further features and advantagesof the invention based on the above-described embodiments. Accordingly,the invention is not to be limited by what has been particularly shownand described, except as indicated by the appended claims. Allpublications and references cited herein are expressly incorporatedherein by reference in their entirety.

What is claimed is:
 1. A receiver for processing data that is received,comprising: means for removing a modulation from the data; means fordetermining an inversion decision for each cluster of the data, whereinthe inversion decision is determined from a marker embedded in the data;and means for applying a rotation to a data symbol in each cluster inaccordance with the inversion decision, wherein the receiver receivesthe data in accordance with an orthogonal frequency divisionmultiplexing method.
 2. The receiver of claim 1, wherein the means fordetermining the inversion decision comprises means for computing a teststatistic for each cluster.
 3. The receiver of claim 2, wherein the teststatistic is quantized.
 4. The receiver of claim 3, wherein the teststatistic is quantized in a manner to provide a binary detection scheme.5. A receiver for processing data that is received, comprising: meansfor removing a modulation from the data; means for determining aninversion decision for each cluster of the data, wherein the inversiondecision is determined from a marker embedded in the data, wherein theprocessing element is configured to determine the inversion decision bycomputing a test statistic for each cluster, wherein the test statisticis quantized and a sequence of the inversion decisions for a pluralityof clusters is decoded to a nearest sequence, wherein the nearestsequence comprises a walsh sequence; and means for applying a rotationto a data symbol in each cluster in accordance with the inversiondecision.
 6. A receiver for processing data that is received,comprising: means for removing a modulation from the data; means fordetermining an inversion decision for each cluster of the data, whereinthe inversion decision is determined from a marker embedded in the data,wherein the processing element is configured to determine the inversiondecision by computing a test statistic for each cluster, wherein thetest statistic is quantized and a sequence of the inversion decisionsfor a plurality of clusters is decoded to a nearest sequence, whereinthe nearest sequence is determined in accordance with a hammingdistance; and means for applying a rotation to a data symbol in eachcluster in accordance with the inversion decision.